Upper-Limb Prosthetics: Seeking the Sense of Touch

By Miki Fairley

I feel, therefore I am.

Although philosopher and mathematician Reneì Descartes
actually said, “I think, therefore I am,” feeling, not only
in the sense of emotions but also in the literal fifth sense,
touch, helps us interact with the world around us and integrate
our sense of self—of who we are as complete beings. As researchers
and developers continue in their quest to meld human and
machine into a single functional entity, restoring a sense of touch
opens the door to creating more intuitive, easier to use, and
more efficient prosthetic limbs. An enormous benefit is that
users would not need the intense visual concentration currently
required to operate a myoelectric prosthesis because touch perception
would enable them to know that they were grasping the
object securely but not so tightly as to crush or break it.

Research shows that augmenting the prosthesis with sensory
perception helps users to integrate the device into their “sense
of self ” as a natural part or extension of their own bodies rather
than as an attached, alien object or tool, thus increasing acceptance
of the prosthesis and promoting a positive body image.
These are important factors, considering the historically high
rejection rates for upper-limb prostheses, ranging from a bit
less than 30 percent to as high as an estimated 50 percent.

“Tactile feedback is essential to intuitive control of a prosthetic
limb, and it is now clear that the sense of body self-identification
is also linked to cutaneous touch,” Paul Marasco, PhD, a
researcher and sensory neurophysiologist at the U.S. Department
of Veterans Affairs (VA) Advanced Platform Technology (APT) Center, Cleveland, Ohio, and his colleagues
point out in the article, “Robotic Touch Shifts Perception
of Embodiment to a Prosthesis in Targeted
Reinnervation Amputees” (Brain. 2011 March; 134
(3):747–58).

Claudia Mitchell, who underwent TMR surgery, became the first person with an amputation to undergo the TSR procedure. Photograph courtesy of the Rehabilitation Institute of Chicago.

The authors investigated the rubber hand illusion,
first used by researchers in 1998. As the subject looks
at the rubber hand, the researcher strokes the subject’s
own hidden hand. The subject begins to perceive the
location of his or her hand shifting toward the rubber
hand. Marasco and colleagues tested two individuals
with amputations who had undergone targeted muscle
reinnervation (TMR) surgery, using a robotic touch
interface coupled with a prosthesis. “Measurements
provide evidence that the illusion [of the prosthesis
perceived as their natural hand] created is vivid….
[T]his may help amputees to more effectively incorporate
an artificial limb into their self image, providing
the possibility that a prosthesis becomes not only a
tool, but also an integrated body part.”

TMR Surgery Leads to Serendipitous Discovery

A hotbed of upper-limb prosthetics
research is bubbling around the
world as scientists and developers
work to add touch sensations to prosthetic
hands.

Jesse Sullivan made headlines around the world when he became the first person with an amputation to use a mind-controlled
prosthesis after undergoing TMR surgery. Photograph courtesy of the Rehabilitation Institute
of Chicago.

One promising initiative resulted
from an unexpected side effect of
TMR surgery at the Rehabilitation
Institute of Chicago (RIC), Illinois.
The TMR procedure involves transferring
residual-arm nerves to alternative
muscle sites. After the nerves
successfully reinnervate the new site,
these muscles can produce electromyogram
(EMG) signals to control
a prosthesis by thought. In 2006,
the technique provided the surgical
platform that enabled Jesse Sullivan,
who has bilateral upper-limb amputations, to become the
first “bionic man” to use a mind-controlled
prosthesis successfully.

Then another exciting development
occurred unexpectedly. A few months
post-surgery, as an assistant wiped
Sullivan’s chest with alcohol, Sullivan
felt the sensation of cold—on his missing
hand. “The nerves we were using
to innervate the muscles were mixed
nerves—a combination of efferent
fibers sending motor signals to the
muscles and afferent fibers receiving
sensory information,” explains Jon Sensinger, PhD, director of RIC’s Prosthesis Design & Control Laboratory. “We were surprised that the afferent
fibers also reinnervated the skin.”

Sensinger

With the opportunity to add sensory input to a
prosthesis, the RIC team developed targeted sensory
reinnervation (TSR). Patients can perceive varying temperatures,
sharpness of objects, vibrations, and pressures
on their reinnervated skin as though these stimuli
were occurring on their missing limb. “We were excited
about this because it gave us a direct sensory portal to
the hand.”

The team evaluated portal stability and quality of sensations.
“Even several years post-TSR surgery, patients
still had the same, consistent sensory portal,” Sensinger says.
The quality of the signal was lower in the hand but higher in
the arm, “which makes sense because we only had the sensory
receptors that were in the arm, not the high density of receptors
in the hand, but we did have the added power the brain’s cortex
uses to process hand sensations.”

To convey sensation, the team used tactors, which did an
excellent job of delivering various sensations gathered by sensors
in the socket, such as pressure, temperature, and force
and delivered to the reinnervated skin and thence to the brain,
Sensinger says. However, much to the team’s surprise, patients
didn’t find the sensory input particularly helpful. “So we took a
step back at that point.”

The researchers have recently taken up the
sensory input challenge again. The team says it
is excited about electrical stimulation research
at Case Western Reserve University (CWRU),
Cleveland, Ohio. “There’s a group led by Dustin
Tyler that is developing some really innovative
protocols,” Sensinger comments, explaining that
Tyler’s research is producing sensation that actually
feels realistic rather than the tingling sensation
usually produced by electrical stimulation.
“It’s a promising approach, and we’re looking
forward to collaborating with them in providing
the sensory portal to the missing hand.”

Pressure sensors on the prosthetic
hand enable the subject to feel when he grasps an object and how tightly he is holding it. Test subjects have successfully completed the Box and Blocks manual dexterity test in which users pick up blocks from one side of a box and place them in the other while their vision is occluded and white noise blocks out prosthesis motor auditory cues. Photographs courtesy of Dustin Tyler, Cleveland VA Medical Center and Case Western Reserve University.

Another angle is determining which type of
sensory feedback is most important to prosthetic
users. “We found that we cannot use a variety of
sensory feedback in our real-time manipulation
of objects because it’s simply too slow,” Sensinger
explains. “What we now believe is that the
brain uses sensory feedback to form models of
how the world works.” The brain uses inverse
dynamic models to learn, for instance, how to
ride a bicycle. Once the skill is learned and the
model is created, people can ride a bicycle easily.
“If feedback is removed, models can’t be generated
as easily.”

Using computational motor control, a multidisciplinary
research approach to discover the
principles of human motor control,
researchers have achieved considerable
success in describing how able-bodied
persons interact with their world, Sensinger
points out. As research continues,
he says, “We’re hopeful that we will
be able to describe how persons with
amputation generate those models and
what type of sensory feedback, such as
pressure or force, would be most useful
to them.”

DARPA Marches Forward

Under the aegis of the Defense Advanced
Research Projects Agency (DARPA)
Reliable Neural-Interface Technology
(RE-NET) program, innovative projects
in advanced prosthetic limbs have used
both brain interfaces and muscle and peripheral nerve interfaces.
Currently, brain interface research is limited to persons
with paraplegia, but peripheral nerve interface technology may
soon be available to individuals with amputation for advanced
prosthetic control.

“The novel peripheral interfaces developed under RE-NET are
approaching the level of control demonstrated by cortical interfaces
and have better biotic and abiotic performance and reliability,”
says Jack Judy, PhD, DARPA program manager, as quoted
in a DARPA press release May
30. “Because implanting them
is a lower risk and less invasive
procedure, peripheral interfaces
offer greater potential than penetrating
cortical electrodes for near-term treatment of amputees.
RE-NET program advances are already being made available to
injured warfighters in clinical settings.” Among other initiatives,
research at RIC, CWRU, and Northwestern University, Chicago,
has been included under the RE-NET program.

Stimulating Real-Life Touch

The gradations in the natural hand’s ability to perceive touch
sensations involved in feeling textures, vibrations, movements, temperatures, pain, discomfort, pressure,
and 3D shapes verge on the infinite.
However, the sensation generally
experienced in current sensory-input
prostheses has been described more
as a tingle, buzz, or poke—still a long
way from the natural hand.

Tyler

The research group led by Dustin
Tyler, PhD, associate professor,
Department of Biomedical Engineering,
CWRU, mentioned by Sensinger,
is taking a new approach to electrical
stimulation by fine-tuning the information cutaneous sensory
nerves perceive and transmit to the brain to create a natural
sense of touch as the prosthesis is used to manipulate objects.
Besides enabling a much more natural sense of touch, the
CWRU team’s efforts have enabled amputee subjects to control
a prosthesis successfully without intense visual concentration
on its movements. Test subjects have even been able to successfully
pick up and move objects without being able to see them.

The research team is using flat interface nerve electrodes
(FINEs) which flatten out nerve fibers so that several of them
can be exposed at once to electrical currents providing feedback,
explains Jeff Blagdon in a May 31 article in The Verge, an
online technology and innovation
magazine (www.theverge.com). The
electrodes are implanted around
the nerves rather than within them,
thus preventing nerve damage.

Vikram Pandit, who has congenital limb deficiency, demonstrates the BioTac tactile sensor with a prosthetic hand. Photograph courtesy of SynTouch.

“The location around the nerves
determines where [the subject]
feels it; how we stimulate determines
what is felt,” Tyler explains.
“One way can make him feel as if
someone is laying a finger across
his hand; with another, he’ll feel
a vibration as though he were running his fingers across the
teeth of a comb. Other stimulation has felt like different surface
textures such as sandpaper or Velcro®. So he has a variety of
sensations depending on how we do the stimulation.” Pressure
sensors also are positioned on the prosthetic hand so that the
amputee subject feels when he grabs an object and how tightly
he is holding it. He has also been able to distinguish between
stimulated sensations and the phantom sensations he experiences,
Tyler notes.

To test the effect of sensation without visual cues, subjects
have successfully completed the Box and Blocks Test for manual
dexterity in which users pick up blocks from one side of a
box and place them in the other, while their vision is occluded
and white noise blocks out prosthesis motor auditory cues.
“Without the sensation, he just moves the prosthesis and closes
it as though he were picking up a block and moving it,” Tyler
says. “When we turn the sensation on, he knows whether or not
he has that block.” The research team has also employed other
tests, such as pulling one grape off a bunch of grapes. “You
don’t want to squeeze or crush the grape, yet you need to hold it
tightly enough to be able to pull it off the stem.”

BioTac is a tactile sensor system consisting of a rigid core containing the electronics, surrounded by a silicone skin filled
with a fluid to give compliance. Photograph courtesy of SynTouch.

What Tyler would like to do soon is partner with some manufacturers
to make a prosthetic hand with the sensors built in.
Once that is available, users can take the electric stimulator home
and use it on a daily basis rather than in short laboratory sessions.
“We want to show that results are repeatable and that it’s generally cost-effective and workable for upper-limb amputees.

Then we want to move into a complete system that can be integrated
with their prosthesis and is on all the time,” Tyler says.

SynTouch: Reflexive Touch Control

Possibly close to beginning commercialization is a product from
SynTouch, Los Angeles, California, a startup company begun in
2008 by researchers from the Medical Device Development Facility
of the University of Southern California, Los Angeles. The
company’s flagship product, the BioTac®, is a tactile sensor system
consisting of a rigid core surrounded by an elastic skin filled with
a fluid to give a compliance “remarkably similar to the human fingertip,” according to the company. The sensors, electronic
circuitry, and connections are protected inside
the core; no sensors are placed in or on the skin itself.

SynTouch researchers tested a commercially available
myoelectric prosthetic hand modified to include
BioTac® sensors. Compared to the subject’s usual
myoelectric prosthesis, the BioTac contact-detection
method resulted in faster completion times to grasp
and move a set of fragile objects.

SynTouch will present a paper
detailing research results during
November’s 2013 International Conference
on Intelligent Robots and Systems
(IROS) in Tokyo, Japan.

Loeb

The BioTac sensors are rather expensive,
sophisticated devices primarily
used for research and robotic applications,
points out SynTouch CEO and
founding partner Gerald E. Loeb,
MD. For prosthetic application, the
team found that most of the benefit
was achieved by incorporating a control that was largely subconscious,
setting off reflexive responses in grasping objects. The
biologically inspired algorithm allowed the user to grasp fragile
objects securely by generating a simple EMG signal.

When the subject was fitted
with tactors, “he actually
sensed all the sensory inputs
from the BioTac, but felt they
were too distracting,” says
Matt Borzage, SynTouch
founding partner and head of
business development. “What
he just really wanted to do was
to be able to grasp different
objects easily with his prosthesis.”
With just the reflexes the
subject was able, without concentrated
visual attention, to
grasp a variety of fragile objects
without breaking, crushing, or
dropping them, according to
Borzage.

Borzage

Working from these results,
SynTouch has developed a simpler, reduced-function version, the
NumaTac, which it exhibited during the May 2013 Institute of
Electrical and Electronics Engineers (IEEE) International Conference
on Robotics and Automation (ICRA) in Karlsruhe, Germany.
“The contact-detection algorithm can be integrated into
an existing prosthetic hand system simply by intervening in the
signal that comes from the myoelectric electrode detectors and
modulating that system as it goes into the controller,” Loeb says.
“We don’t have to make any changes to the hardware and software,
which is very helpful because of the regulatory requirements
involved in medical device engineering.”

For these and other new prosthetic technologies to enter
the clinical mainstream and benefit patients, they must navigate
the complex and often lengthy road to commercialization.
Prosthetic component manufacturers keep an eye on these
developments and, for promising projects, can help shepherd
technologies into the marketplace. Ottobock, headquartered in
Duderstadt, Germany, for example, “has many collaborations
worldwide and is always evaluating new technologies that have
the potential to provide benefits to our patients,” says Kevin
Kelley, international project coordinator for Ottobock Healthcare
Products, Vienna, Austria. “It is a long and difficult road
from innovative concepts to commercial production of medical
devices, and so we often work with innovators to guide this
process. At the end we go through a tough and thorough evaluation
of the resulting technology, looking at technical, competitive,
legal, and financial aspects in order to determine whether
it makes sense to incorporate [it] into a commercial product.”
As research and development teams from many different disciplines
push back the boundaries of prosthetics in the quest to
replicate the natural hand, individuals with upper-limb amputations
and O&P clinicians can expect exciting developments
to come.

Miki Fairley is a freelance writer based in southwest Colorado. She can be contacted via e-mail at